1. Field of the invention:
This invention relates to (1) a variable interferometric device, which comprises a Fabry-Perot interferometer provided with a pair of substrates facing each other with a space therebetween, wherein the substrates are displaced by an external force so as to vary the interferometric characteristic of said interferometric device, (2) a process for the production of the variable interferometric device, and (3) a process for the control of the interferometric characteristics of the variable interferometric device. This invention further relates to an optical sensor using a variable interferometric device, the interferometric characteristics of which vary with external physical quantities.
2. Description of the prior art:
There are spectrometers provided with a diffraction grating. The diffraction grating is mechanically rotated, so that each of the optical elements of the spectrometers must be disposed with high precision, causing difficulties in the enlargement thereof.
On the other hand, Fabry-Perot interferometric devices using a piezoelectric element therein can operate as a spectrometer that has no mechanical operating parts. The optical disposition of each element of the Fabry-Perot interferometric device is easily performed, but the production of the interferometric devices and the control of the interferometric characteristics thereof are extremely difficult, which causes difficulties in the practical use thereof. FIG. 20 shows a conventional Fabry-Perot variable interferometric device, in which a pair of transparent plates 500 and 501 are disposed in a parallel manner by means of a holder 502 provided with a hole 503 through which light beams pass. A reflecting film is coated on the surface of each of the plates 500 and 501 which face each other. A piezoelectric element 504 furnished with electrodes on both surfaces thereof is attached to the peripheral area of the plate 501, so that the piezoelectric element 504 expands and contracts depending upon the application of a driving voltage to the piezoelectric element 504, thereby allowing a variation in the distance between the plates 500 and 501 (i.e., the distance between the reflecting films). The variation in the distance between the reflecting films makes possible the changes in the interferometric characteristics of the device, so that the distance between the reflecting films must be set with strict precision and must be controlled with strict uniformity. Therefore, the holder 502 must be manufactured with extremely high accuracy, which causes difficulties in the production of a Fabry-Perot interferometer. Moreover, due to the thermoexpansion of the holder 502 based on changes in temperatures in the atmosphere, there is a possibility that the distance between the reflecting films will vary. The control of the distance between the reflecting films by means of the piezoelectric element is also extremely difficult.
As mentioned above, in the conventional Fabry-Perot interferometer device, the distance between the reflecting films depends upon the mechanical accuracy of the holder 502, which causes many problems.
Conventional optical sensors using a small-sized and inexpensive light source such as a light-emitting diode (LED), etc., are designed such that light from a light source, which can emit light with a relatively wide range of wavelengths, passes through a first Fabry-Perot interferometric device, the characteristics of which vary with the physical quantity of the object to be measured, and through a second Fabry-Perot interferometric device, the characteristics of which do not vary with the physical quantity of the object, and reaches a photodetector in which its optical intensity is changed into an electrical signal. Although the conventional optical sensors using two Fabry-Perot interferometric devices detect physical quantities to be measured as a variation in the amount of light, the optical intensity at the photodetector varies with changes in light-emission intensity and/or scatter of connections losses arising when optical devices such as optical lenses, optical fibers, etc., are inserted into this optical measuring system, so that a stable-sensor output power can not be obtained by the conventional optical sensors.